FIG. 6 shows a cross-sectional view of a package of a prior art plastic molded semiconductor integrated circuit device. In FIG. 6, the reference numeral 1 designates a plastic such as epoxy resin, the numeral 2 designates a metal lead frame, the numeral 3 designates a pewter such as Au or Ag. The numeral 4 designates a silicon chip, the numeral 5 designates a bonding pad such as aluminum, the numeral 6 designates an integrated circuit section, the numeral 7 designates a silicon chip protection film such as silicon nitride, and the numeral 8 designates a bonding wire.
The prior art plastic molded semiconductor integrated circuit device fails to be waterproof and has a problem of generating .alpha.-rays. Recently, however, the waterproofness thereof has been enhanced by reducing the impurity ions in the plastics and enhancing the adhesive characteristics of the plastics 1 against the silicon chip 4 and the lead frame 2. Furthermore, silica which has only a small number of .alpha.-ray generators is used as fillers for the plastics, and the reliability of such a plastic molded integrated circuit device is enhanced to a great extent (refer to articles "Plastic moldings where the damp-proofness is steadily enhanced", NIKKEI Electronics, Sept. 14, 1981, No. 273, pp. 118 to 135, and "The reliability of the semiconductor plastic molding", Technical Search Report of Electronics and Communication Society (reliability), March 1983, R82-81, pp. 23 to 28).
Recently, however, the oversizing of the integrated circuit chip and the miniaturization of the pattern width and pattern interval are rapidly enhanced, and if the metal pattern wirings inside the semiconductor integrated circuit transform due to the stress of the molding plastics, there occurs a short circuiting between the metal pattern wirings, resulting in a fault in the semiconductor integrated circuit device (refer to articles; "A slide phenomenon of a vapored aluminum wiring in the thermal bombardment of the plastic molded semiconductor element", Technical Search Report of Electronics and Communication Society (reliability), October 1979, R79-23 pp. 57 to 64, and "Epoxy sealing material for VLSI advancing in the lowering of stress", NIKKEI Electronics Micro Devices June 11, 1984, pp. 82 to 92).
FIGS. 7(a) and 7(b) show an elevational view of a normal metal pattern wiring, and the cross-sectional view of FIG. 7(a) on line VIIb to VIIb, respectively. FIGS. 8(aand 8(b) show an elevational view of the metal pattern wiring which is transformed by the stress of the plastics, and the cross-sectional view of FIG. 8(a) in line VIIIb to VIIIb, respectively. In both Figures, the reference numeral 11 designates a metal pattern wiring such as aluminum or molybdenum. The numeral 12 designates a chip protection film such as silicon nitride. The numeral 13 designates an insulating film such as silicon oxide. The numeral 14 designates a semiconductor substrate such as silicon. The numeral 15 designates a plastics such as epoxy resin. As shown in FIG. 8(b), if there occurs a short-circuiting between the metal pattern wirings 11 due to the stress of the plastics in the direction of Z, this, of course, leads to a malfunction of the integrated circuit.
The reason why the metal pattern wiring transforms as a result of the stress of the molding plastics is that the plastic molded semiconductor integrated circuit device is constituted of substances having different elasticity and different thermal expansions such as plastics 1, lead frame 2, and silicon chip 4 as shown in FIG. 6. A distortion of the plastics 1 is caused by a change in the temperature environment ocurring in the plastic molding process or after the product is completed and is transmitted to the metal pattern wiring of the integrated circuit section 6 through the chip protection film 7, thereby transforming the metal pattern wiring.
Such transformation of the metal pattern wiring is likely to occur not at the central portion of the integrated circuit, but at the peripheral circuit, especially at the four corners of the silicon chip and a portion of straight wiring extending from one edge of the chip to the other edge thereof. This is considered to be caused by the difference in thermal expansion between the plastics and the silicon chip being large close to the end of the device, and that a straight wiring is weak against stress applied from the vertical direction. And thus, it is generally considered that the short-circuiting caused by the transformation of the metal pattern wiring happens less when the metal pattern wiring is made rough near the periphery. Under such construction, however, the chip size becomes large, resulting in a costly device.
Furthermore, as a countermeasure against the transformation of the metal pattern wiring caused by the stress of the plastics, it has been attempted to lower the stress of the plastics by purification of the plastics itself, or to conduct a coating over the silicon chip surface by plastics such as polyimid before molding the silicon chip surface by plastics. However, both of these lead to increased processes and poor mass production capabilities, resulting in a costly device.